The process and apparatus of the present invention relate to the rapid production of expanded mesh sheet, e.g., the expansion of a portion of metal strip contained within unexpanded imperforate border portions for the manufacture of grids, particularly grids used in pasted battery electrodes.
In conventional preparation of strip by rotary slitting to permit lateral expansion which forms a network of meshes, e.g., the method shown in FIGS. 8 and 9 of U.S. Pat. No. 1,472,769, longitudinally aligned slits arranged in a staggered relation may be cut into a strip by engagement between a pair of opposed rolls, each roll comprising spaced discs having circular cutting edges interrupted by recesses which are spaced circumferentially. On rotation of the rolls, recesses in the discs of one roll become aligned with like recesses in the discs of the other roll to leave spaced unslit portions of strip which become nodes having bonds that join adjacent strand-like components in the staggered relation. In this slitting operation, the discs of one roll press alternate strand-like components out of the plane of the strip in one direction while the discs of the other roll press intervening strand-like components out of the plane of the strip in the opposite direction. Some bending and stretching of bonds connecting adjacent strand-like components occur in this operation, and during lateral expansion to provide a network of meshes, to impose additional forces on the bonds. If the network of meshes is to be contained within unexpanded imperforate border portions, preforming or elongation of the strand-like components is required to compensate for shortening during expansion of the slit area relative to the border portions. Otherwise irregular ripples will form in the imperforate borders or these borders will require corrugation to provide uniform shortening to the length of mesh portion. In the slitting, preforming and lateral expansion in successive steps of strip material, particularly strip material having low tensile strength such as lead or lead alloys, bonds between strand-like components may become points of weakness which can be prone to failure during subsequent use.
The method of U.S. Pat. No. 1,472,769 provides zones of expanded metal between non-expanded longitudinal strips or ribs without shortening the slit portion of strip. Strip is slit as previously described, then flattened into the original plane. Flattened strip moves between corrugating rolls which coact to press alternate series of bonds, i.e. rows of bonds connecting laterally adjacent components, out of the plane of the strip. This causes strands to be stretched and to be inclined laterally. Divergence of the side strips opens meshes which are then flattened. It is considered undesirable to stretch the strands by a single operation to a sufficient length to accommodate a full lateral expansion of the meshes. Use of successive corrugating and expanding operations overcomes resilience of the metal. Final flattening of the expanded meshes sets the corrugated bonds and inclined strands in a common plane.
In the method of U.S. Pat. No. 1,212,863, stretching of components in a slit portion of strip is effected by moving alternate nodes of each component out of the plane of the strip. Successively acting dies provide both slitting and stretching as the strip advances. Since the dies act successfully, the strip must be moved through the assembly intermittently.
U.S. Pat. No. 1,205,299 illustrates, in FIG. 3, feed rolls comprising discs which taper into relatively thin peripheral edges made up of uniform and regularly alternating notches and gripping projections. These rolls, although similar in longitudinal section to slitting and preforming rolls of the present invention, are used solely to help control the movement of strip which has been slit and expanded intermittently in a preceding die press.
Elongation of strands preslit by rotary cutting and subsequent lateral expansion often leads to breakage of the strands due to excessive stretching or to failure of the strands at node bonds due to concentration of stresses at the nodes.
In the forming of meshes from strip by reciprocal techniques, e.g. the method of U.S. Pat. No. 1,482,600, stretching of bonds between adjacent strand-like components occurs during progressive slitting and opening of meshes using reciprocating cutting tools wherein individual wire segments are cut from strip and stretched out of the plane of the strip by the reciprocating cutters. Twisting of nodes during deformation by the reciprocating cutting tools and subsequent bending as meshes are brought into the plane of expanded sheet impose stresses on the nodal bonds joining the strand-like components. Product having imperforate borders may be obtained by the reciprocating technique but this method is sensitive to changes in strip material dimensions which can adversely affect final dimensions and is limited in speed of production.